Demodulators or coherent carrier trackers, particularly for phase shift keyed signals, have two basic problems during acquisition; one theoretical, one practical.
When acquiring a modulated signal in a demodulator with a control loop bandwidth that is narrow compared to the modulation rate, the loop may very likely synchronize to a subharmonic of the modulation rate above or below the desired carrier frequency synchronization point. This can occur in high gain loops whenever the channel or the demodulator is band limited, i.e., wherever the modulation transitions are no longer instantaneous. Mathematically and physically the subharmonics are generated in the demodulation process of sufficient magnitude for the loop to synchronize to one of them. Using an unmodulated carrier during synchronization is one way of preventing this possibility. This means the control loop must have DC response.
Frequently it is undesirable practically to use DC coupled stages. AC coupling is used with chopper stabilization to generate DC response. The loop now may acquire improperly on the subharmonics of the chopper repetition frequency just as though the signal were modulated. Means exist to sense these improper lock conditions and to sequence to the proper lock state at an increase in acquisition time. However, in many applications this increase in acquisition time is unsatisfactory.